1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and more particularly, to a method of fabricating a static random access memory (SRAM) cell. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for realizing the high packing density and cell stabilization of the static random access memory cell.
2. Discussion of the Related Art
Since a static random access memory (hereinafter referred to as SRAM) does not require refreshing of the memory and its operation timing is easily adjustable, it is capable of having the same access time and cycle time as a microcomputer. Further, the SRAM can be operated as fast as a random access memory (RAM). Accordingly, SRAMs are widely used in buffer memories of large size calculators, main memories in supercomputers, and control memories.
SRAMs using a flip-flop are classified into Enhancement/Depletion (E/D) type SRAMs, complementary metal oxide semiconductor (CMOS) type SRAMs, and high resistance-load type SRAMs.
A conventional SRAM cell will be explained with reference to the accompanying drawings.
FIG. 1 and FIG. 2 are an equivalent circuit diagram and a layout of a conventional SRAM cell, respectively.
Referring to FIG. 1, first and second drive transistors TD1 and TD2 are based on a unit cell of a flip-flop, and gates of the first and second drive transistors TD1 and TD2 are connected to "B" and "A" points, respectively. As shown, in each of the unit cells, the first and second access transistors TA1 and TA2 are respectively connected to bit-lines B/L1 and B/L2, and their gates are connected to the word-line W/L.
Referring to FIG. 2, a layout of a conventional SRAM cell has a complicated structure because active regions are formed having irregular shapes. That is, an SRAM cell includes active regions irregularly formed in a limited region, gate electrodes of first and second access transistors formed in predetermined portions of the active regions, and gate electrodes of first and second drive transistors. The gate electrode of the first access transistor and the gate electrode of the second drive transistor are connected to each other on a drain region of the first access transistor. Also, the gate electrode of the second access transistor and the gate electrode of the first drive transistor are connected to each other on a drain region of the second access transistor. The gate electrode of the second drive transistor is connected to the drain of the first drive transistor through a first contact hole. The gate electrode of the second drive transistor is connected to a load resistance R1 through a second contact hole. At this time, the first and second contact holes are located in the same line.
Operations of the conventional SRAM will now be explained with reference to FIG. 1 as follows.
When a high signal is applied to a word-line W/L, the first and second access transistors TA1 and TA2 are turned on, so that the unit cell is electrically connected to the bit-line. When the first drive transistor TD1 is off and the second drive transistor TD2 is on, the data of the bit-line B/L2 is transmitted to a memory cell through the second access transistor TA2. In contrast, when the first drive transistor TD1 is off, the bit-line B/L1 has no path for current and is charged-up by the first access transistor TA1. In other words, when a high level data is written in the memory cell, a voltage larger than a threshold voltage is applied to the word-line so that the first and second access transistors TA1 and TA2 are turned on. The high level data is thus applied to an "A" point. Since the electric potential of the A point is a high level, a high signal is applied to the gate of the second drive transistor TD2 connected to the A point, and the second drive transistor TD2 is turned on. In contrast, a low level data of the bit-line B/L2 is applied to a "B" point, so that the first drive transistor is turned off.
Subsequently, in reading data, the bit-lines B/L1 and B/L2 become equi-potentials and a voltage larger than the threshold voltage is applied to the word-line, thereby turning on the first and second access transistors TA1 and TA2. Accordingly, when the data is the high level at the A point, the electric potential of the bit-line B/L1 is increased. Conversely, when the data is the low level at the A point, the electric potential of the bit-line B/L1 is decreased.
FIG. 3 is a cross-sectional view showing a structure of a conventional SRAM. The conventional SRAM includes a field oxide layer 42 to define first and second active regions on a first conductivity type semiconductor substrate 41, a gate electrode 43 of an access transistor on the first active region, and a gate electrode 44 of a drive transistor spaced by a predetermined distance from the gate electrode 43 covering the field oxide layer 42 and on the second active region, source and drain impurity regions 46 and 46a having a second conductivity type in the substrate 41 at both sides of the gate electrode 44 of the access transistor, an insulating layer 47 having a contact hole exposing the gate electrode 44 of the drive transistor on the entire surface including the gate electrode 43 and 44, a load resistance layer 50 electrically contacting the gate electrode 44 of the drive transistor over the insulating layer 47, a ground line 48 between the load resistance layer 50 and the gate electrode 44 of the drive transistor insulated from the load resistance layer 50 and the gate electrode 44 of the drive transistor, and a bit-line 52 contacting a source impurity region of the second conductivity type through the contact hole.
A conventional method of fabricating an SRAM cell having the aforementioned structure will be explained with the accompanying drawings as illustrated in FIGS. 4A to 4D.
Referring to FIGS. 4A, a first polysilicon layer is formed on the entire surface of a first conductivity type semiconductor substrate 41 having active regions defined by a field oxide layer 42. POCl.sub.3 is doped into the first polysilicon layer. Next, a photoresist layer (not shown) is coated on the first polysilicon layer to define gate electrode regions of an access transistor and a drive transistor. The first polysilicon layer is selectively removed by a photo-etching process to form gate electrodes 43 and 44 of an access transistor and of a drive transistor. Then, with the gate electrodes 43 and 44 serving as masks, impurity ions of the second conductivity type are lightly doped into the substrate 41.
Subsequently, a first insulating layer is formed on the entire surface including the gate electrodes 43 and 44 and then is etched-back to form sidewalls 45 on both sides of the gate electrode 43 of the access transistor. With the sidewalls 45 serving as masks, impurity ions having the second conductivity type are heavily doped to form source and drain impurity regions 46 and 46a having a lightly doped drain (LDD) structure. In this process, the portion, which is the boundary of the active region and the gate electrode 44 of the drive transistor, indicated as "A" in FIG. 4A is formed by the diffusion of doped POCl.sub.3 into the first polysilicon layer.
Referring to FIG. 4B, a second polysilicon layer 47 is then formed on the entire surface including the gate electrodes 43 and 44, a second insulating layer 47. Subsequently, a photoresist layer is coated on the second polysilicon layer to define a power-line. Next, the photoresist layer is patterned with a photo-etching process to form a ground line 48.
Referring to FIG. 4C, a third insulating layer 49 is formed on the entire surface of the second insulating layer 47 including the ground-line 48. Subsequently, the second and third insulating layers 47 and 49 are selectively removed to form a contact hole exposing the gate electrode 44 of the drive transistor. Then, a third polysilicon layer is formed and then patterned to electrically contact the gate electrode 44 of the drive transistor to form a load resistance layer 50.
Referring to FIG. 4D, a planarizing insulating layer 51 is formed on the entire surface of the third insulating layer 49 including the load resistance layer 50, and then is partially removed to expose a portion of the surface of the substrate 41 and form a contact hole over the source impurity region 46 of the access transistor. Next, a metal layer is formed on the entire surface including the planarizing insulating layer 51 and then patterned to form a bit-line 52, thereby completing the overall process of a conventional SRAM cell.
However, a conventional SRAM cell and a method of fabricating the same have the following problems.
Since active regions, as shown in the layout in FIG. 2, are unsymmetrically formed, drive transistors are also formed unsymmetrical. As a result, it is very difficult to stabilize the data in the cell. Further, limitations in the packing density due to the layout also limit the width of a drive transistor.